1. Field of the Invention
The present invention relates to a speed control system for a working vehicle equipped with a hydrostatic transmission for propelling the vehicle.
2. Description of the Prior Art
There are many types of working vehicles equipped with a hydrostatic transmission, in which a control shaft for varying a swash plate angle of the hydrostatic transmission is directly and mechanically connected to a change speed pedal through a rod or the like. The swash plate of the hydrostatic transmission receives a reaction force of oil delivery from a hydraulic pump for returning the hydrostatic transmission to a neutral position. The change speed pedal must be operated against this force, which requires a relatively strong pedal operating force. Meanwhile, a working vehicle has been proposed in which a control position of the change speed pedal is electrically detected with a potentiometer or the like, and the swash plate angle of the hydrostatic transmission is controlled by means of a motor in response to the detected control position.
A working vehicle such as an agricultural tractor engages in an operation while running on an unpaved road, on a soft field, or in other unfavorable conditions. Thus, mud, soil, water and the like tend to adhere to lower positions of the tractor. Under such conditions, the electric linkage as noted above is not expected to assure a high degree of reliability and durability, but rather likely to cause a malfunction.